A conventional image reader comprises an array of light-emitting diodes (hereinafter referred to as "LEDs") arranged in a straight line to form a light source, a rod lens array for guiding a reflectied light from a document, an array of photosensing elements arranged in a straight line for receiving the reflected light from the document to generate a photocurrent, and a signal detection circuit connected to the photosensing element.
When the image reader reads the document, all LEDs of the LED array are turned on at the same time so as to illuminate the document. The rod lens array guides the reflected light to each photosensing element. Each photosensing element is responsive to the reflected light to generate a photocurrent. The signal detection circuit detects a signal by means of the photocurrent when it is in a charge storage mode. Therefore, the signal detection circuit needs to provide the same numbers of switches as the numbers of the photosensing elements, so that the circuit structure is complicated. Moreover, a power consumption of the image reader is very large because all the LEDs are turned on at the same time during reading.